Many commercially available vaso-occlusive coils have capped ends which are formed by the simple procedure of heating the coil end sufficiently to liquify the composite metal into a round cap-like artifice. Although the rounded cap provides a surface which is relatively minimal when contacting the internal surface of a delivery catheter, we believe that the tip could be improved upon, at least from the aspect of coil delivery.
Some coils appear to have lost a measure of flexibility in the region near the tip, perhaps because of the heat necessary to melt the metal at the adjoining tip. This short region of stiffness produces a leg which presses against the lumen of the catheter, at least until the tip clears the distal end of that catheter. The energy stored in pushing the coil through the distal end of the catheter causes the coil to jump forward and the catheter to retract as the coil leaves the catheter. If a very precise placement of the catheter tip is desired, e.g., where a small-necked aneurysm is accessed, such a lurching and slipping is particularly not desirable.
Out intent in this invention is to improve the stiffness characteristics of the vaso-occlusive coil so to enhance the ease with which the coils advance through the catheter, improve the handling of the coil as it exits the catheter, and improve the coil's ability to be deployed gently as it leaves the catheter.
Finally, the flexible tip promotes the position stability of the coil during placement. The "droop" of the flexible distal section tends to engage the vessel wall and press the trailing stiffer section of the coil against the opposing wall. The resulting coil mass is formed more quickly and more compactly.
Our solution to this problem is to assure that at least one end of the vaso-occlusive coil is somewhat more flexible than the adjoining midsection. The leading end of the coil, i.e., the end of the coil which is distally placed, is most important, although for practicality's sake, it is desirable that both ends be so constructed. In such a way, the coil may be introduced from the catheter in either direction into the blood system. There are several ways to increase the flexibility of these end regions: vary the diameter of the wire making up the coil, change the spacing of the coil turns, vary the diameter of the coil, and change the inherent properties of the material in the wire, such as by annealing.
This technique is useful whether using coils which have electrolytic or mechanical detachment links at their ends, or when using coils having attached thrombogenic fibers. The technique is especially useful on coils having secondary shapes when those coils are relaxed. There are other helical coil devices having varying pitch spacing or the like. For instance, U.S. No. Pat. No. 485,652, to Pfingst, issued Nov. 8, 1892 describes a car spring--apparently a railroad car spring--in which the diameter of the rod making up the coil gradually tapers. The inner diameter of the spring appears to be of constant diameter throughout. It is obviously quite stiff.
U.S. No. Pat. 4,553,545, to Maass et al., shows a intravascular prothesis made up of a helical spring having a variable pitch. The device is intended to hold a human body lumen open and consequently is fairly stiff.
U.S. No. Pat. 4,760,849 shows a planar blank intended for the manufacture of a coil spring. The coil spring is suitable for a translumenal implantation. The device is either used as a stent to hold a vascular lumen open or it may be used as a blood filter. The coil spring filter may be used as a vena cava inferior filter to prevent the formation of emboli and their passage into the lung.
U.S. No. Pat. 4,830,023, to de Toledo, shows a medical guidewire having a coil tip. The device has a degree of flexibility and a tip region of greater flexibility. The coil making up the helically wound spring is in two pieces: one having a greater pitch than the other.
Similarly, U.S. No. Pat. No. 5,069,217, to Fleischhacker Jr., shows a coil of varying pitch soldered to the end of a guidewire combination.
U.S. No. Pat. No. 5,171,383, to Segaye et al. shows a guidewire having varying flexibility along the axis. The flexibility is varied by changing the heat treatment temperature along the length of the guidewire.
None of these publications show the concept described herein in which at least a portion of the center of the vaso-occlusive device is less flexible than at least one of the ends.